1. Field of the Invention (Technical Field)
The present invention relates to an apparatus and method for passively compensating for thermal expansion, and particularly to an apparatus for regulating the distance separating two objects using regulating components fashioned from materials having a non-constant coefficient of thermal expansion.
2. Background Art
Changes in ambient temperature cause objects to undergo physical changes, especially in size. Changes in the size of an object due to temperature changes is a function of the coefficient of thermal expansion (CTE) of the material of which the object is composed. In some engineering and scientific operations, changes in size of an object can be deleterious. For example, an ambient temperature change may alter the size of one or more key components of a device, thus impairing the function or accuracy of the device, especially if the device is a finely tuned or calibrated machine. For instance, it is well known that the imaging performance of an optical system is dependent on temperature. Increases and decreases in ambient temperature change the physical dimensions of both the lens elements as well as the mechanical components of an optical system. Also, the refractive properties of the lens elements change with changes in ambient temperature.
It is seen, therefore, that there is a need for a method and apparatus for adjusting or controlling the spacing between two objects, such as components in a device, to compensate for changes in component dimensions due to shifts in ambient temperature. Such an apparatus or method ideally is passive, that is, the compensation occurs xe2x80x9cautomaticallyxe2x80x9d as a consequence of the temperature change, as a opposed to requiring active monitoring and intervention of the machine operator.
One method and apparatus for passively compensating for thermal expansion or contraction, to control component spacing, is disclosed in U.S. Pat. No. 5,557,474 to McCrary, the entire disclosure of which is incorporated herein by reference. The McCrary device uses a plurality of annular spacers with conical seats. The spacers are arranged in pairs, with spacers in a pair having different CTEs, and all the spacers ordinarily having a different CTE from the segments of the regulated device (e.g., a lens barrel). As the ambient temperature rises, all of the spacers, as well as all of the mechanical elements of the system, expand. However, one spacer in each pair expands considerably faster then the other. The spacers being annular cones, the first spacer moves outward relative to the central axis of the optical system faster than the other spacer. Thus, unwanted positional shifts between to objects, such as lenses, are compensated during the thermal change. Materials with differing CTEs and angled interfaces transform a transverse or radial dimensional change into a longitudinal dimensional change to control relative movement as a function of temperature, thereby maintaining, for example, the focus of a compound lens system.
The McCrary device may be adequate to its purpose when the device is not exposed to an extremely wide range of temperatures, and for spacer components fashioned from a material with a relatively non-constant CTE over the temperature range encountered. The McCrary device is inadequate when the ambient temperature may range widely, and especially when the spacer components are made from materials having a non-constant CTE. It is known that many materials otherwise acceptable for spacer construction have a non-linear CTE at the extremes of a given temperature range. Additionally, in many applications, it is desirable to use compensating spacer elements that are manufactured from a material, such as a polymer, having a certain critical performance characteristic (coefficient of friction, shear modulus, modulus of elasticity, hardness, or the like), and yet also having a non-linear CTE in the anticipated ambient temperature range.
It is known, therefore, to provide a collection of compensating components made from materials having constant CTEs over the encountered temperature range to provide xe2x80x9cautomaticxe2x80x9d adjustment for thermal changes. However, in some optical systems, it may be necessary to employ a compensating element manufactured from a material having a very high CTE in order to provide adequate compensation over the expected temperature range. Materials having high CTEs commonly also have non-linear CTEs, that is, the CTE itself changes as a function of temperature. A material with a sufficiently high CTE (or perhaps some other characteristic) to meet application requisites may have a CTE sufficiently non-linear as to be unusable in known compensation systems.
A continuing need remains, therefore, for an apparatus and method for providing passive compensation for thermal expansion or contraction in a device, that provides compensation over an appreciable temperature range and especially incorporating elements composed of materials having non-constant coefficients of thermal expansion. Some optical systems preferably or necessarily incorporate plastic components having non-linear coefficients of thermal expansion, i.e., the coefficient itself changes significantly as the ambient temperature changes. With such non-linear dimensional changes, the compensations provided by a xe2x80x9clinearxe2x80x9d device such as McCrary""s are insufficient.
An apparatus for passively compensating for undesirable positional shifts in objects due to temperature changes. The apparatus is particularly suited for use in optical assemblies, where it is desired to compensate for thermal changes in the system to maintain an optical focus despite large changes in environmental temperature. A specially configured compensating member is placed between two objects whose spacing is to be regulated. The compensating member is composed of a material having a non-linear coefficient of thermal expansion, to permit the use of a compensating member also having a very high average coefficient of thermal expansion. The compensating member has multiple ramps or faces positioned at differing angles to permit thermal compensation to be predetermined and provided for selected temperatures, despite the non-linearity of the compensating member""s coefficient of thermal expansion.
There is provided according to the invention an apparatus for compensating for thermal expansion or contraction of two objects, the apparatus comprising: a member having a nonlinear coefficient of thermal expansion, the member comprising: a first face disposed at a first angle in relation to the axis and defining a first oblique seat, the magnitude of the first angle corresponding to the member""s coefficient of thermal expansion at a first average temperature; and a second face adjacent to the first face, the second face disposed at a second angle in relation to the axis and defining a second seat, the second angle corresponding to the member""s coefficient of thermal expansion at a second average temperature. The member and the first and second seats preferably are substantially annular. The recited first angle is greater than the second angle, and the first face defines an inner ring seat, the second face defines an outer ring seat, wherein the outer ring seat is radially outward from the axis in relation to the inner ring seat. The apparatus preferably further comprises a second inner ring seat and a second outer ring seat on an opposite side of the member from the inner ring seat and the outer ring seat.
The invention may comprise a thermally compensative optical system comprising: a forward lens barrel and a second lens barrel arranged along an axis, the barrels movable axially in relation to each other; a member disposed between and contactable with the forward and second lens barrels, the member having a non-linear coefficient of thermal expansion, and the member comprising: a first face disposed at. a first angle in relation to the axis and defining a first seat; and a second face adjacent to the first face, the second face disposed at a second angle in relation to the axis and defining a second seat. The first seat preferably comprises a first inner oblique seat and the second seat comprises a first outer oblique seat, wherein the outer oblique seat is radially outward from the axis in relation to the inner oblique seat. This embodiment preferably further comprises a second inner oblique seat and a second outer oblique seat on an opposite side of the member from the first inner oblique seat and the outer oblique seat. Preferably, the second inner oblique seat comprises a face disposed at a third angle in relation to the axis, and the second outer oblique seat comprises a face disposed at a fourth angle in relation to the axis. Also the third angle preferably is approximately equal to the first angle, and the fourth angle is approximately equal to the second angle, and the member and all the seats are substantially annular.
In this same embodiment of the invention, the first angle preferably is greater than the second angle, and the first face defines an inner ring seat, the second face defines an outer ring seat, wherein the outer ring seat is radially outward from the axis in relation to the inner ring seat. The system preferably further comprises: a forward inner compensation seat on the forward lens barrel; a forward outer compensation seat on the forward lens barrel; a rear inner compensation seat on the second lens barrel; and a rear outer compensation seat on the second lens barrel, wherein the forward inner compensation seat is contactable with the first inner oblique seat of the member, and the forward outer compensation seat is contactable with the first outer oblique seat of the member, and the rear inner compensation seat is contactable with the second inner oblique seat of the member, and the rear outer compensation seat is contactable with the second outer oblique seat of the member.
A primary object of the present invention is to provide a method and apparatus for compensating for thermal expansion and contraction in an assembly, using compensating elements having a non-linear coefficient of thermal expansion.
A primary advantage of the present invention is that passive, selective, thermal compensation is provided in a system or assembly exposed to a wide range of temperature changes, despite the desirability to employ compensating elements having a non-linear coefficient of thermal expansion.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.